1. Field of the Invention
This invention relates to an optical information recording and reproducing apparatus, and more particularly to a tracking control apparatus and method wherein a light beam accurately trace signal tracks formed on a recording medium to include a wobbled area.
2. Description of the Prior Art
Recently, an optical recording medium, an optical magnetic recording medium and so on have been developed as a recording medium for recording various information such as video and audio information, etc. and is commercially available in the market. Such an optical recording medium includes a read-only type disc such as CD, CD-ROM, DVD-ROM, etc., a write-once-read-many type disc such as CD-R, DVD-R, etc., and a rewritable type disc such as CD-RW, DVD-RAM, etc.
In reproducing the optical disc, a light beam must be irradiated onto the center of a track recorded with an information to trace the track at a constant speed so that a photo detector may accurately detect a variation in a light quantity reflected by the track and further an accurate information may be reproduced from electrical signals from the photo detector. In this case, a converged state of a light irradiated onto the track is controlled in accordance with a distribution of a reflected light irradiated onto the photo detector. Also, a tracking state is controlled in accordance with a distribution of a reflected light irradiated onto the photo detector. Electrical signals converted by means of the photo detector are processed to thereby detect a rotation speed of the optical disc, which is controlled in accordance with the detected rotation speed. Further, since a reproducing position on the optical disc, that is, a track position onto which a laser beam is irradiated is seen from the reproduced information, the optical disc is accessed in a random basis.
However, since a recording disc has not been recorded with signals as described above, it must be manufactured to have such a signal track structure that is capable of recording an information. The signal track must be formed to have land and groove pattern in such a manner to indicate a tracking state, and support signals indicating a rotation speed of the disc are written into the signal track. Also, a physical position information is written into the signal track so that a random access of the recording optical disc is available at the time of recording.
Accordingly, in an optical disc such as CD-R, CD-RW and so on, as shown in FIG. 1, there are formed signal tracks 2 and 4 continued in a spiral shape and having land and groove patterns. In such an optical disc, groove signal tracks 4 only are usually used as tracks for recording an information. The groove track 4 includes an wobbled area 1 in which a certain period of wobbling signal is preformatted. The wobbling signal includes an address and the like indicating the physical position of a signal track. A rotation speed information of the optical disc and a recording position information of the optical disc, that is, an information about a position of the signal track onto which a laser beam is irradiated, are obtained from the wobbling signal on the wobbled area 1.
Likewise, in a DVD-RAM, as shown in FIG. 2, signal tracks 6 and 8 having land and groove patterns are intermittently formed. These signal tracks 6 and 8 is divided into data sectors which each consists of a head field and a recording field. The head field is provided with prepit stream 12 and 14, and the recording field is provided with land or groove racks 6 or 8. Each prepit stream 12 and 14 include a sector identification code so as to indicate the physical position of the data sector. The first prepit stream 12 indicates the physical position of the land signal track 12 and the second prepit stream 14 indicates the physical position of the groove signal track 14. Wobbled areas, in which a certain period of wobbling signals are preformatted in the same phase, are formed at each side of the land or groove signal track 6 or 8.
In order to allow a light beam to trace the center of land and groove tracks defined in a recording medium, there has been widely used a tracking control apparatus which is classified into three beam system and one beam system called "pushpull system". Most information recording and reproducing apparatus have employed the tracking control apparatus of one beam system because the tracking control apparatus of one beam system is capable of simplifying a configuration of optical pickup as well as enhancing a light efficiency at the time of recording in comparison to the tracking control apparatus of three beam system. advantages of the one beam system tracking control apparatus will he apparent from the following description with reference to FIGS. 3A and 3B.
Referring to FIG. 3A, the one beam system tracking control apparatus includes a spindle motor 18 for rotating an optical disc 16, an optical pickup 19 for accessing the optical disc 16, and a tracking error detector 20 and a tracking servo 26 that are connected, in series, to the optical pickup 19. The optical pickup 19 includes a beam splitter BS for guiding a light beam from a laser diode LD to an objective lens OL and a light beam from the objective lens OL to a photo detector PD, and an actuator AC for moving the objective lens OL in the up, down, left, and right direction to perform a focusing and a tracking. The objective lens OL converges a light beam progressing from the beam splitter BS to the optical disc 16. The beam splitter BS allows a light beam from the laser diode LD to be irradiated, via the objective lens OL, onto land signal tracks 2 and 6 or groove signal tracks 4 and 8 of the optical disc 16, and allows a light beam reflected by the optical disc 16 to be progressed to a photo detector PD. The actuator AC moves the objective lens OL in the left and right direction in such a manner that a light beam trace the center lines of the land signal tracks 2 and 6 or the groove signal tracks 4 and 8. The photo detector PD converts a reflective light quantity received, via the objective lens OL and the beam splitter BS, from the optical disc 16 into an electrical signal. As shown FIG. 3B, the photo detector PD consists of four photo detecting pieces PDa, PDb, PDc and PDd so as to detect a distribution of a light irradiated on the signal tracks 2 and 6, or 4 and 8. The tracking error detector 20 receiving electrical signals from the four photo detecting pieces PDa, PDb, PDc and PDd includes a differential amplifier 22 for detecting a difference signal between a sum signal of electrical signals from two photo detecting pieces PDa and PDd positioned at the left side on the basis of a track direction and a sum signal of electrical signals from two photo detecting pieces PDb and PDc at the right side, and a low pass filter(LPF) 24 for making a low pass filtering of the difference signal from the differential amplifier 22 and applying the filtered difference signal to the tracking servo 26 as a tracking error signal Te. The tracking servo 26 receiving the tracking error signal Te from the LPF 24 responds to the tracking error signal Te to control a current signal or a voltage signal applied to the actuator AC, thereby moving the objective lens OL in the left or right direction, that is, in the outer or inner circumference direction of the optical disc in such a manner that a light beam traces the center line of the track.
According to the tracking control apparatus of one beam system as described above, however, since a direct current(DC) offset is generated as the objective lens OL is moved in the left or right direction, it is difficult to allow a light beam to trace the center line of the signal track. This DC offset is generated when the center of the objective lens OL departs from the optical axis as shown in FIGS. 4A and 4C; while it is not generated when the center of the objective lens OL falls at the optical axis as shown in FIG. 4B. More specifically, since a light beam reflected from the optical disc 16 departs from the center of the photo detector PD when the center of the objective lens OL deviates to the left and light from the light axis as shown in FIGS. 4A and 4C, a distribution of a light irradiated to two left photo detecting pieces PDa and PDd and two right photo detecting pieces PDb and PDc of the photo detector PD does not become symmetrical. Due to this, a DC offset is generated at the tracking error signal. On the other hand, when the center of the objective lens OL is positioned at the light axis as shown in FIG. 4B, a light axis of a light beam reflected from the optical disc 16 meets the center of the photo detector PD, so that a distribution of a light irradiated to two left photo detecting pieces PDa and PDd and two right photo detecting pieces PDb and PDc of the photo detector PD becomes symmetrical. Accordingly, a DC offset is not generated at the tracking error signal. In view of this, the one beam system tracking control must be improved so that it can effectively compensate for a DC offset generated when applied to a recording medium having wobbled areas.